LETTER REPORT: MEAN CURRENT FLOW ACROSS THE MOUTH OF
THE CAPE FEAR RIVER TIDAL INLET AND THE NEW CHANNEL REGION, 2002

Dr. Jesse E. McNinch
Department of Physical Sciences
Virginia Institute of Marine Science
College of William and Mary

Methodology

Acquisition System:

600 kHz Broadband RDI Acoustic Doppler Current Profiler

Differential GPS (Northstar)

Mean currents were measured across the mouth of the Cape
Fear River tidal inlet and the seaward portion of the ebb tidal delta around
the new shipping channel using a ship-mounted ADCP. The location of the two
survey transects are shown on Figure 1. The vessel steamed continuously around
each transect for over 13 hours, making a complete loop every hour or less.
This technique provides a measure of current magnitude and direction at
discreet locations along the transect every hour and spans the periods of the
primary tidal constituents (M2, S2). Other variables that typically force
currents in tidal inlets, such as wind-driven flows and river discharge, are
also incorporated within the 13-hour snapshot of currents. Each transect was
run within several days of the predicted spring high tide and wind conditions
prior to the surveys were light and did not likely play a significant role in
the measured flows. A long-term time series of currents and water level around
the inlet would best determine the relative influence the various tidal
constituents and meteorological forces (wind, discharge) may play. We believe,
nonetheless, that the transect measurements reflect near maximum magnitudes for
astronomical flows and the spatial patterns seen across the transects fairly
characterize recurring flow directions under similar conditions. Furthermore,
the spatial coverage provided by the ADCP transects is well suited to provide
calibration/verification of the modeled flows currently being simulated by
USACE personnel.

Measurements were collected using RDIís WinRiver acquisition
software with 1-m vertical bins. No spatial (horizontal) or temporal ensembling
was conducted during acquisition, other than the vertical 1-m binning. A Matlab
script was written to convert the RDI proprietary raw binary file format to
ascii text. Data were processed using Matlab and reduced to horizontal
ensembles every 50 m along each transect. Near-surface and near-bottom averages
were determined from an integration of the 1-m bins throughout the water column
(upper half of the water column and lower half of the water column,
respectively).

Results

Figures 2-17 show the magnitude and direction of flow for
the near-surface (red vectors) and near-bottom (blue vector) every 50 m along
the inlet mouth transect. Several observations are readily apparent. First, as
surmised from the bathymetry of the ebb tidal delta
(see bathymetry report, 2001-2002),
current speed and direction are influenced by the shoals on the
western flank of the delta particularly during flooding periods (transects
00-02; 11-15). Water appears to be funneled through the flood margin channel
flanking Oak Island and around the two linear shoals before joining the main
channel. The transect does not extend far enough seaward to measure possible
flood margin channel flow that may exist in front of Baldhead Island. The large
shoal that extends from Baldhead Island seaward, however, suggests that a
significant tidal flow in the form of a flood margin channel does not occur on
the east side of the inlet. Second, in contrast to the dispersed flow across
the ebb tidal delta seen during flooding periods, much of the ebb flow appears
to be concentrated in the main ebb channel. This funneling of ebb flow through
the ebb channel is consistent with other observation in the tidal inlet
literature. Lastly, current magnitudes consistently exceed 1 m/s (2 kts) and
are highest in the near-surface. Vertical stratification is most apparent
during near slack periods (transect 11).

Cross-sectional flow across the tidal inlet is shown for
each transect in Figures 18-33. Across-channel variations in flow magnitude and
direction is apparent in many transects (e.g. transects 01-03) as well as
vertical stratification. The northern leg of the transects extend across the
entire inlet mouth, including the flood margin channel along Oak Island and the
main ebb channel. Figures 34 and 35 show the measured tidal prism during the
April 13, 2002 survey. These measurements indicate the flow was ebb dominated
during this time, totaling roughly 4x108 cms during the ebb and 3x108
cms during the flood. The tidal prism measured in 2000 before the dredging also
showed an ebb dominance but at a considerably smaller value of just under 2x108
cms. The absolute value of tidal
prism volumes between 2000 and 2002, however, cannot be directly compared
because the initial survey in 2000 only spanned the main shipping channel and
did not extend across the entire inlet mouth. Further limitations to direct
comparisons of volume are the dynamic nature of other forces which influence
flow such as wind-forcing and river discharge as well as differences in
astronomical tides at different times of the year and across a tidal epoch
(i.e. spring tides are not necessarily equal through time). Future surveys will
reoccupy the transect location established in 2002 and thus be more compatible
for direct comparisons of flow patterns around the inlet mouth, particularly
the ebb channel and flood margin channel. Although only a long-term time series
of flow and water level measurements across the region prior to and after
channel dredging would allow definitive observational evidence of changes in the
tidal prism due to dredging, the spatial coverage offered by the ADCP transects
will provide insight into possible changes in flow patterns across the ebb
tidal delta in the future. Most importantly, these observations will serve as
verification of numerical model simulations, which can, in turn, best address
changes due to channel deepening from dredging.

Flow direction and magnitude around the new channel and
portions of the original shipping channel are shown in Figures 36-48. As seen
in the other vector plots, near-surface (red vectors) and near-bottom (blue
vector) flow is plotted every 50 m along the transect. Flow magnitudes are much
reduced in this region as compared to the inlet mouth region. Velocities reach
1 m/s only in the vicinity of the old shipping channel at the height of the ebb
(Figure 42, transect 06). Flow patterns also appear quite similar to those
measured in 2000, prior to excavation of the new channel, except for small
areas directly over the new channel. Interestingly, highest flow magnitudes
appear to remain in the old shipping channel and only during transect 08 is
there any evidence that the flow field is influenced by the new channel where
near-bottom flow appears to lead in the flooding direction.